Conventionally, a ceramic porous body has been used as a filter such as a diesel particulate filter (DPF) or a water treatment filter (for example, refer to Patent Documents 1 and 2). Usually, a ceramic porous body used as a filter has a wall-shaped or plate-shaped portion, and traps a solid particle or the like contained in fluid by causing the fluid to permeate through the portion as a filter layer.
For example, as the ceramic porous body used as DPF, a honeycomb-shaped ceramic porous body (honeycomb structure) having partition walls for defining a plurality of cells is used widely. By plugging ends of cells adjacent to each other alternately (in a checkered pattern) in such a honeycomb structure, a filter capable of trapping a particulate matter (PM) contained in an exhaust gas from diesel engine or the like is obtained.
That is, when an exhaust gas flows in a predetermined cell (inflow cell) from one end in the plugged honeycomb structure, the exhaust gas passes through a porous partition wall, moves to an adjacent cell (outflow cell), and is then emitted. When the exhaust gas permeates through the partition wall, the partition wall functions as a filter layer, and traps PM contained in the exhaust gas.
A ceramic porous body used for such a filter requires high gas permeation performance in order to reduce a pressure loss. Here, as one of means for improving gas permeation performance, it is considered to form a pore in a wall-shaped or plate-shaped portion (for example, a partition wall of a honeycomb structure) serving as a filter layer into a shape of a high aspect ratio, such as an elongated shape, and to orient the pore such that a length direction of the pore is parallel to a thickness direction of the portion. By orienting the pore in the portion in this way, the number of pores communicating in the thickness direction of the portion is increased. Therefore, a permeation path of fluid such as gas is short to obtain high permeation performance.
For example, a pore of a high aspect ratio can be formed by using a pore former of a high aspect ratio in a fiber shape or the like (for example, refer to Patent Document 3). However, it is difficult to orient such a pore former of a high aspect ratio such that a length direction thereof is parallel to a thickness direction of a wall-shaped or plate-shaped portion serving as a filter layer.
That is, a ceramic porous body used as a filter is usually manufactured by forming a ceramic formed body (for example, a honeycomb formed body) having a wall-shaped or plate-shaped formed portion from a raw material for forming using extrusion capable of mass production at low cost and firing the ceramic formed body. A die conventionally used for extrusion of a ceramic formed body has a raw material supply surface and a raw material forming surface opposite to the raw material supply surface. At least one introduction hole for introducing a raw material for forming is provided in the raw material supply surface. On the other hand, a slit (forming groove) for extrusion of a ceramic formed body having a wall-shaped or plate-shaped formed portion from a raw material is provided in the raw material forming surface. The introduction hole communicates with the slit in the die. A raw material introduced into the die from the introduction hole passes through the slit, and thereby becomes a ceramic formed body having a wall-shaped or plate-shaped formed portion in a predetermined thickness (for example, refer to Patent Document 2).
In such a conventional ceramic formed body extrusion method using a die, in a case where a raw material for forming contains a particle of a high aspect ratio (for example, a pore former), when the raw material passes through a narrow slit, the particle is oriented such that a length direction thereof is parallel to an extrusion direction of a formed body. Here, the extrusion direction of the ceramic formed body is a direction perpendicular to a thickness direction of a wall-shaped or plate-shaped formed portion, and is not a direction parallel to a thickness direction of the formed portion. That is, by such a conventional ceramic formed body extrusion method, it is not possible to obtain a ceramic formed body which contains a particle (for example, a pore former) of a high aspect ratio and in which the particle is oriented such that a length direction of the particle is parallel to a thickness direction of a wall-shaped or plate-shaped formed portion.
Note that Patent Document 3 discloses a method for manufacturing a ceramic porous body having a pore oriented almost in one direction using a pore former of a high aspect ratio. However, even by this manufacturing method, the pore former of a high aspect ratio is oriented such that a length direction thereof is parallel to an extrusion direction of a formed body (refer to FIGS. 3 and 6 in Patent Document 3). Therefore, even by using the manufacturing method disclosed in Patent Document 3, it is not possible to obtain a ceramic formed body which contains a particle (for example, a pore former) of a high aspect ratio and in which the particle is oriented such that a length direction of the particle is parallel to a thickness direction of a wall-shaped or plate-shaped formed portion.
In addition, conventionally, a method for manufacturing a precursor of a porous ceramic having a unidirectional through hole derived from a gap between ceramic formed bodies by assembling a plurality of the ceramic formed bodies in one layer and then performing compression molding has been disclosed (refer to Patent Document 4). However, mass production is more difficult and manufacturing cost is higher in this method than in the above extrusion method.    [Patent Document 1] JP-A-2006-225250    [Patent Document 2] JP-A-2005-81609    [Patent Document 3] JP-B-4669925    [Patent Document 4] JP-A-11-139887